Boonton 4240 RF Power Meter - Manual (Page 47)

Boonton 4240 Series RF Power Meter
4. Operation
This section provides detailed background information on various aspects of operation of the Model 4240. It is assumed that
the reader is familiar with the basic operating procedures covered in Section 3. This section covers the following topics:
Sensor calibration, Zeroing, Filtering, Noise, Dynamic range, Measurement time, High frequency accuracy, Chart recorder
operation and Waveform sensitivity.
4.1 Sensor Calibration
General. Two types of calibration are associated with the Model 4240 - instrument calibration and sensor calibration. The
instrument (less sensors) must be calibrated using a stable and accurate DC source, such as the Boonton Model 2510, to
ensure interchangeability of sensors. Instrument calibration procedures are covered in the Model 4240 Service Manual.
Sensor calibration data is of two types: linearity and high frequency calibration factors. Sensor calibration data for up to four
sensors can be stored in nonvolatile EEPROM plus each sensor data adapter contains the data matched to the corresponding
power sensor.
14-Point Linearity Data. Linearity data, also referred to as AC reference frequency linearity data, is supplied with the sensor
and can be manually entered into the non-volatile Tables or Adapters. For sensors ordered with the instrument, linearity data
is stored in the sensor data adapter before the instrument is shipped.
At the reference frequency (50 MHz, or 40, 60, or 94 GHz), each sensor has two gain factors for each range: upscale and
downscale points. Refer to Figure 4-1. The upscale points are in the range of 4000-7000, which is a gain correction factor.
Upscale points are calibrated at the factory at about 70% of full scale. The downscale number is an offset correction at about
25% of full scale. Thus, for a diode sensor (7 ranges), there are 14 points; for thermal sensors there are eight points. Ranges 0
and 1 share the same data points.
AutoCal.
Initiates a multi-point sensor gain calibration of the selected sensor with the internal 50MHz step calibrator. This
procedure calibrates the sensor’s linearity at a number of points across its entire dynamic range. A sensor must be connected
to the channel input.
High Frequency Calibration Points. In addition to linearity data, there are high frequency calibration points. Calibration
points covering the entire sensor frequency range are supplied with each sensor. Below 1 GHz, the sensor response is flat,
and frequency calibration points need not be entered.
The Model 4240 provides space for up to 60 points for each sensor table. Frequency calibration points need not be in equal
frequency increments; however, the entry of data must be done in ascending order of frequency. For both diode and thermal
sensors, a calibration factor of 0 dB is implied at 0.00 GHz so that the instrument may be operated below the first data point.
4.2 Zeroing
The automatic zeroing routine of the instrument takes measurements on the lowest five ranges and applies these as correction
factors on subsequent measurements. Offsets in the sensor and input amplifiers are linearly corrected in the internal software.
Offsets on the highest ranges are below 0.02% of full scale, and do not need correction.
Input power to the sensor must be removed before the zeroing function is executed or an error message will be displayed. The
instrument will perform zeroing, however, if the signal is less than full scale on range 0. This feature provides a great deal of
offset capability for temperature effects without rezeroing the input amplifier hardware.
For full accuracy at low signal levels, power must be removed from the sensor several seconds before zeroing to allow the
sensor to settle. This is especially true if a large signal had been applied to the sensor in the previous 20 seconds or so
because of the dielectric absorption of the capacitors in diode sensors, and because of thermal retention in thermal sensors.
The error resulting from different input conditions can be determined from Figure 4-2 or 4-3, as applicable. The curves in
Operation
4-1